1. Field of the Invention
The present invention relates to the field of photolithography to form integrated circuits and more particularly to the field of photoacid generators used in photolithography.
2. Discussion of Related Art
Photolithography is used in the field of integrated circuit processing to form the patterns that will make up the features of an integrated circuit. A photoresist is employed as a sacrificial layer to transfer a pattern to the underlying substrate. This pattern may be used as a template for etching or implanting the substrate. Patterns are typically created in the photoresist by exposing the photoresist to radiation through a mask. The radiation may be visible light, extreme ultraviolet (EUV) light, or an electron beam. Most photolithography is done using either the “i-line” method or the chemical amplication (CA) method. In the i-line method, the photoresist film is rendered soluble when a photoactive compound (PAC) is irradiated and transformed into a soluble species that may subsequently be removed along with the surrounding matrix by an aqueous developer such as tetramethylammonium hydroxide (TMAH). In the chemical amplification method, the radiation applied to the photoresist causes the decomposition of a photo-acid generator (PAG) to cause the generation of a small amount of acid throughout the pattern of exposed resist. The acid, in turn, causes a cascade of chemical reactions in the photoresist matrix either instantly or in a post-exposure bake. This cascade of chemical reactions increases the solubility of the resist so that the photoresist may be removed by a developer, such as TMAH. An advantage of using the CA method is that the chemical reactions are catalytic and therefore the acid is regenerated afterwards and may be reused, thereby decreasing the amount of radiation required for the reactions and making it possible to use shorter wavelengths of light such as EUV.
The distance diffused by the acid produced by the decomposition of the PAG from where it is decomposed to where it reacts chemically with the polymers that form the photoresist matrix is a key factor in the resolution of the photoresist. This reaction of the polymer with the acid catalyst transforms the insoluble polymers into soluble polymers through the removal of protecting groups attached to soluble groups on the polymer. The shorter the diffusion distance of the acid within the photoresist matrix, the better the resolution of the photoresist, and therefore the smaller the critical dimension (CD) that can be printed and the better the CD control of the structures formed in the photoresist. The strength of the acid that is produced by the decomposition of the PAG is also a factor in the performance of the photoresist. The stronger the acid, the more efficient the deprotection of the chemically amplified polymer and ultimately the better the sensitivity of the photoresist. Therefore, it is valuable to have PAG's that are in close proximity to the chemically amplified polymer and that will efficiently produce strong low diffusion acids when the PAG decomposes. PAG's that will produce strong and large low diffusion acids include perfluorooctyl sulfonate (PFOS) and perfluoroalkyl sulfonate (PFAS.) But, PFOS has known toxicity, and PFAS, being smaller and lower in volume than PFOS, has a longer diffusion length than PFOS. Also, fluorine is significantly less transparent to EUV light and therefore presents a performance liability when used with EUV wavelengths. Additionally, PFOS and PFAS are long but narrow straight chained molecules that are substantially limited in volume. Other strong acids including, for example, bis(fluorosulfonamide)'s, bis(fluorosulfoxide)methides, alkylsulfonates, fluoroalkylsulfonates, and fluoroantimonates have been used in place of PFOS and PFAS, but do not offer the same resolution due to their lower acidity and differing dipole moments.